There are some example virtual host definitions in the httpd-vhost.conf file that are there to show you what the syntax looks like. They look like this:

# VirtualHost example:
# Almost any Apache directive may go into a VirtualHost container.
# The first VirtualHost section is used for all requests that do not
# match a ServerName or ServerAlias in any <VirtualHost> block.
#
<VirtualHost *:80>
ServerAdmin webmaster@dummy-host.example.com
DocumentRoot "/usr/docs/dummy-host.example.com"
ServerName dummy-host.example.com
ServerAlias www.dummy-host.example.com
ErrorLog "/private/var/log/apache2/dummy-host.example.com-error_log"
CustomLog "/private/var/log/apache2/dummy-host.example.com-access_log" common
</VirtualHost>

Add # signs to the beginning of those lines to comment them out.

An alternative to this is to use an app like VirtualHostX to do this for you ($35).

Edit 5 Dec 2011:

Here's a new blog post by the makers of Alfred on how they set this up: